1. Field of the Invention
This invention relates generally to the field of thermal management. More particularly, it relates to ammonium carbamate-based thermal management systems and methods for managing thermal loads.
2. Description of the Related Art
Thermal management of heat in the temperature range of 20-100° C. is a problem encountered in a variety of situations and is particularly problematic for high-flux heat and in volume- and weight-constrained environments. Conventional methods of thermal management such as heat rejection, heat conversion, and heat storage are generally insufficient to manage this low-grade or low quality and/or high-flux heat. High quality heat is generally understood to mean that the operational temperature of the entity requiring cooling is significantly higher than the temperature of the heat sink to which the thermal energy is transferred. In contrast, low quality heat occurs when the fluid or object requiring cooling is close in temperature to the heat sink. Low-grade waste heat generated from machinery, electrical equipment, and electronic devices presents a significant problem as most equipment and electronics must operate in a temperature range of about 20-100° C.
Heat rejection transfers or “rejects” excess thermal energy to the environment via radiation, thermal convection (gaseous or liquid cooling fluids), or thermal conduction through a solid medium. Heat conversion captures waste heat and recovers some of the energy by, for example, heating water for use in a steam engine to generate electrical power. Both of these methods generally work best with a large temperature difference, so thermal management of low quality heat using heat rejection and/or heat conversion is often poor because the waste heat is only slightly above the heat rejection temperature.
Thermal storage is a means of thermal management in which heat is absorbed into a storage medium for later reuse or eventual rejection to the environment. Two classes of thermal storage materials are common in practice: those based on latent heats and those based on sensible heats. Latent heats are those associated with phase changes, such as the enthalpy (heat) of melting of a paraffin wax, while sensible heats are associated with temperature increases, such as heat stored in water as the water temperature is raised. To compensate for the low rate of heat transfer encountered with low quality heat, the surface area of interaction may be increased, the quantity of coolant fluid may be dramatically increased, and/or heat spreaders such as metal matrices may be employed. For terrestrial applications in which land is readily available, these constraints generally do not pose insurmountable problems. However, in volume- and/or weight-constrained environments, these compensations are frequently insufficient, and the increasingly complex systems are often impossible to implement.
Furthermore, many latent heat-based thermal management materials introduce additional complications when implemented. For instance, lithium hydroxide stores a significant quantity of thermal energy upon melting, which makes it a good thermal storage medium. However, the resulting liquid can be difficult to constrain, and the reverse process can require an unacceptably large amount of time, limiting the overall system availability and duty cycles. Likewise, paraffin wax-based systems are limited by the slow crystallization rate of the liquid wax. Application of many thermal management systems is further limited due to their dependence on gravitational forces. Many phase change materials lose contact with heat spreaders upon melting under certain gravitational conditions. This loss of thermal contact due to liquefaction frequently renders the waxes ineffective for thermal management.
Thermal management systems based on chemical reactions are currently being explored. Metal hydride-based systems such as Mg/MgH2 and LiAlH4 have high gravimetric thermal energy densities and produce large amount of hydrogen that can be stored or used in other processes. However, these systems share a number of drawbacks that make their use impractical in many environments. Hydrogen is extremely flammable, and most metal hydrides react violently with water, both of which present significant safety hazards. In addition, metal hydride-based systems are problematic in that the reaction kinetics are often slow, with poor reversibility, and they have a high desorption temperature.
Current systems employing ammoniates likewise suffer from numerous drawbacks that often make their use impractical or impossible in some environments. Some systems combine ammonia gas and a salt such as NaCl or KCl to form a complex, which then endothermically disassociates. However, formation of the ammonia-salt complex requires high pressures and large amounts of salt. Other ammonia-based systems simply take advantage of the phase change of ammonia from a liquid to a vapor and either eject the ammonia gas into the environment or circulate the ammonia gas through large radiators to reject the heat. For example, COIL (Chemical Iodine Oxygen Lasers) lasers are generally cooled by flash evaporation of liquid ammonia, whereas ammonia-based heat pipes are utilized for thermal management on spacecraft. The liquid ammonia used in these systems is toxic and must be carefully handled and stored, particularly in enclosed or confined spaces such as aircraft and spacecraft. Tank leakage or spillage of the ammonia can quickly result in lethal concentrations of corrosive ammonia gas.